The following relates to automated decoupling and tuning of radio frequency (RF) resonance coils. It finds particular application with coils that receive and/or transmit RF signals in magnetic resonance imaging, and will be described with particular reference thereto. However, it is to be appreciated that the invention may also find application in decoupling and tuning of signals in magnetic resonance spectroscopy and the like.
During the transmission and receipt of signals in a magnetic resonance environment, various techniques are employed to improve image quality. Such techniques include the decoupling/detuning of coils, noise adjustment of preamplifiers, mutual decoupling of coil elements, retuning for the purpose of multi nuclei operation, load dependent fine tuning of resonant frequencies, and radio frequency (RF) shimming of magnetic resonance coils. In magnetic resonance (MR), surface and local receive coils commonly use positive-intrinsic-negative (PIN) diodes for decoupling and coupling during transmission and reception, respectively. Typically, the PIN diode of the receive coil is biased such that the coil is detuned or decoupled from the resonance frequency during resonance excitation. For signal reception, the PIN diode is biased such that the surface coil is tuned to the resonance frequency. Other solid-state elements can also be used to switch between the tuned receive mode and the detuned mode.
This kind of detuning has several disadvantages. First, the detuning lines to surface coils are prone to electromagnetic interference. A significant problem is coupling of RF to these detuning lines during transmission. In high field systems, common mode RF resonances on these cables may cause local heating of the patient. As a result, the detuning lines are insulated for the relatively high reverse bias which adds to the bulkiness of the coil cable. Cable bulkiness problems are compounded in multi-element surface coil arrays. If the detuning lines of many surface coil elements are bundled in one cable, the forward currents add and may cause B0 inhomogeneities. Moreover, the Lorentz force on the cable may be disturbing.
During manufacture, whole body and other coils are tuned to the Larmour frequency of the system. The tuning is accomplished by manually adjusting tunable capacitors distributed around the coil while loaded with an average patient in a standard position. In a bandpass birdcage type coil, for example, there is typically one or more adjustable capacitors in each of the 8 to 32 rods and a like number of adjustable capacitors in each end ring. Other whole body coils, such as SENSE coils, also have complex and time consuming tuning issues. Each capacitor is manually adjusted to tune the mode spectrum, a lengthy and costly iterative process. In addition, when a patient is introduced into the assembled scanner, the mode spectrum can change (e.g., by 400 kHz in a 3 T body coil) based on variation in the size and position of the patient. Due to the manual tuning process, it is not feasible to tune the whole body coil whose capacitors are in the interior of the scanner construction on a patient by patient basis.
For systems with field strength of 3 T and more, the wavelength of the RF within the patient is in the same order of magnitude as the spatial dimensions of the patient. This results in RF resonance effects within the patient leading to B1 inhomogeneities. Moreover, the elliptic shape of a patient leads to different loads for different portions of the body coil resulting in additional B1 inhomogeneity. It has been shown that these effects can be compensated by adjusting various distributed capacitors within a whole body coil or resonator. However, utilizing conventional manual means, most distributed capacitors within the whole body coil are not readily accessible for manual adjustment.